With power converters, it may become necessary to convert excess energy into heat. There can be many situations where this is necessary. For example, an energy source (such as a wind power station) may be connected to a power converter to supply energy to the power converter. However, an energy sink connected to the power converter may be unable to take up excess energy (e.g. a power supply grid in case of a short circuit) or that the power converter may be incapable of feeding energy back into the grid because of its design. In power converters with a voltage source inverter, this could lead to an excessive increase of the voltage source and thus, the power must be lowered.
It is also possible for multi-point power converters under certain operating conditions (depending on load, control system and design) to develop non-symmetric voltages in the capacitors that are connected in series of a voltage source inverter. In this case, it may also become necessary to convert energy into heat. Furthermore, there may also be other operational reasons for lowering voltages in intermediate circuit capacitors at a certain point in time and to convert the energy stored in them into heat.
The task to convert excess energy into heat is typically taken up by brake choppers. Generally, brake choppers include a power semiconductor switch that can be turned off and a power resistance that is connected to DC voltage connections of a voltage inverter. There have been some attempts aimed at lowering the cost of semiconductor switches for the brake chopper circuit. For example, various circuits may be connected to the output of a voltage inverter and therefore can operate with semiconductor switches that cannot be turned off as long as DC voltage is applied at the power converter output. Other methods suggest a circuit that is connected in parallel to a semiconductor switch of a voltage inverter that is capable of operating with semiconductor switches that cannot be turned off. Other attempts suggest supplementing the actual three-phase inverter with brake resistances and to operate it as brake chopper.
One conventional brake chopper has a modular design with distributed brake resistances. By allocating the brake resistance to a power electronics module, it is possible to achieve a modular structure and distribute the braking performance into several such modules. However, semiconductor switches are generally necessary for controlling the brake chopper circuit which is needed in addition to the semiconductor switches in the power converter in order to ensure the functioning of the brake chopper. As a result, the prevention or reduction of the excess voltage adds complexity to the circuitry and increases costs. Therefore, an improved electrical circuit that facilitates the prevention or reduction of excess voltage within a power converter would be useful.